Methane is formed thermogenically during burial and thermal maturation of coal and/or other hydrocarbons or it may be produced biogenically by the action of microbes on a carbonaceous feedstock. Bacteria are considered to be the primary degraders of compounds in a coal feedstock in, an anoxic and reducing environment, producing a range of intermediates which are successively degraded to small molecule methane precursors, such as, hydrogen gas, carbon dioxide, acetate and various other compounds (e.g., dimethyl sulfide, formate, methanol and methylamines). These precursors are then converted to methane via microbial consortia which comprise methanogenic archaea and include various other bacterial populations. This methanogenic process may occur via a number of mechanisms including CO2 reduction, acetoclastic (from acetate) processes and methylotrophic processes. In this way, carbon from the feedstock enters the carbon cycle to ultimately end up in methane generated in the formation. Due to macronutrient limitations, natural biogenic methane production is slow and occurs over long time-scales. Production from a typical coal seam methane (CSM) well may occur for 5-7 years, after which time, the rate of production generally becomes uneconomic and the well may be abandoned.
Given the potential to recharge reservoirs that are undersaturated in methane, stimulating in-situ biogenic methanogenesis through biostimulating nutrient amendment of formation waters (biostimulation) and/or microbial consortia (bioaugmentation) can have significant economic potential. Laboratory studies have demonstrated that some coal seam formation waters, after biostimulating nutrient amendment, degrade coal feedstock better leading to significant methane generation rates. However the potential for methanogenesis can vary considerably from well to well, even within the same coal basin.
Despite this, when methanogenic consortia in a formation are well supported by a favourable environment such that the methanogenic microbes flourish beyond their natural state, the microbes may become stimulated to convert carbonaceous media feedstock into methane gas at significantly better than natural speeds. In such optimised systems, enhanced methanogenesis rates may result in commercially interesting methane production levels.
Over time however, the rate of methanogenesis gradually lessens until the methane generation rate slows or ceases and the methane recovery process becomes no longer commercially viable. At this point, approaches to re-stimulate or re-invigorate satisfactory rates of methanogenesis become important.
As the rate of gas generation tends to decrease with time, it is believed that this may indicate that only a proportion of the coal feedstock might be available for biological degradation and that plateaus in the gas generation could be attributed to the depletion of “bio-available” coal feedstock. Another hypothesis is that organic intermediates formed during the process and/or the presence of large quantities if biostimulating nutrients added during biostimulation could ultimately be toxic to methanogens and therefore inhibit methanogenesis over time.
U.S. Pat. No. 6,543,535, the relevant contents of which are incorporated herein by reference, provides a process to enhance methane generation from oil deposit feedstocks, such as heavy oils or tar, that are left over after primary and secondary oil recovery processes. The left over oil is typically microtrapped or adsorbed onto mineral surfaces and is unsuitable for typical recovery techniques. The process describes involves methodically analysing the microbial consortia and its subterranean environment to determine the changes in the ecological environment required to promote microbial generation of methane from the oil based feedstock. As the hydrocarbon feedstock in liquid/oil type deposits is materially different to the carbonaceous porous feedstocks described herein, nutrient adsorption and/or desorption processes occurring, if any, are expected to be very different to those observed with a coal feedstock, for example. In short, this document teaches a process for stimulating the activity of pre-existing microbial consortia in a subterranean formation to convert liquid type hydrocarbon feedstock to methane. The process involves analysis of the fluid and rock of the formation, determination of the presence of microbial consortia and characterisation thereof. This information, together with the information obtained from the analysis of the fluid and rock, is used to determine an ecological environment that promotes in-situ microbial degradation of formation hydrocarbons and promotes microbial generation of methane by at least one methanogenic microorganism of the consortia. This information is then used as the basis for modifying the formation environment to produce methane through improved support of microbial consortia particular present by adapting the environment conditions to better suit the consortia present.
U.S. Publication No. 2010/0081184 teaches prophetic methods for optimisation of methane production in a subterranean hydrocarbon formation. Optimisation of methane production is achieved by employing a mathematical model describing the geological, geophysical, hydrodynamic, chemical, biochemical, geochemical, thermodynamic and operational characteristics of systems/processes for the in-situ bioconversion of carbon bearing subterranean feedstocks to methane. Optimisation methods include the introduction of microbial nutrients, methanogenic consortia, chemicals and electrical energy, as required. This document recognises that the amount and rate of bioconversion is a function of several factors, including the specific microbial consortia present, the nature or type of the carbon-bearing formation, the temperature and pressure of the formation, the presence and geochemistry of the water within the formation, the availability and quantity of nutrients required by the microbial consortia to survive and grow, the presence or saturation of methane and other bioconversion products or components, etc. In particular, the document proposes that efficient bioconversion requires optimised delivery and dispersal of nutrients into the formation, the dispersal of microbial consortia across the surface area of the formation, the exposure of as much surface area of the formation to the microbial consortia as possible, and the removal and recovery of the generated methane, carbon dioxide and other hydrocarbons from the formation. To this effect, the methods purposefully increase and maintain the pressure within the subterranean formation well above its initial condition, such that the flow of fluids, nutrients, microbial consortia and generated methane, carbon dioxide and hydrocarbons is optimised. Thus, enhancing nutrient bioavailability focuses on improving the formation's porosity/permeability characteristics, as well as internal and fracture surface area characteristics to improve the bioavailability of ex-situ supplied nutrients by providing better access to microbes by increasing available surface area/penetration as well as ease of removal of gaseous products from the formation.
It should further be noted that a range of factors can inhibit the biogenic conversion of coal to methane. While biostimulating nutrients are important to support and promote biological activity and growth, such amendment can stimulate activity in biostimulating nutrient deficient circumstances, there is an optimal biostimulating nutrient concentration where the beneficial effect is maximised; above this concentration biological activity decreases and eventually the nutrient concentration becomes toxic. Furthermore, since expensive biostimulating nutrients are a key aspect of biostimulation, their consumption relative to the quantity of methane generated will determine the ultimate economic benefit.
It will be appreciated that the subterranean formation environment supporting methanogenesis comprises a number of specific components effecting methanogenesis. These include the particular indigenous microorganism consortia present and their specific biostimulating nutrient and energy consumption requirements, a formation's specific structural components, for example, rock, sand and/or sediment, each having a particular mineralogy, porosity, density, etc., as well as associated chemical and physical properties. Other relevant components may include formation pressure, temperature, etc. Furthermore, associated formation water has a number of relevant chemical and physical properties, for example, pH, conductivity, etc.
It will be appreciated that some aspects of the subterranean formation environment are constrained and cannot be altered, for example, geology, mineralogy, while other variables can be modified relatively easily, for example, formation water biostimulating nutrient concentration, pH, salinity, etc.
Understanding the interplay between the constraints and variables of any formation under investigation with respect to biogenic methanogenesis allows the optimum methanogenesis environment to be determined. Where environmental amendments necessary to promote and/or sustain the activity of the microbial consortia can be identified and applied to a formation via a formation specific environment amendment regime, implementation of such amendments may improve methanogenesis. As discussed in U.S. Pat. No. 6,543,535 and U.S. Publication No. 2010/0081184, By controlled modification of key environmental parameters to support consortia, methanogenesis of carbonaceous media to methane can be initiated, promoted and/or maintained over extended period of time.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
An object of a preferred embodiment of the invention is to make more efficient use of expensive methanogenesis biostimulating nutrients so that reduced amounts of, and in some embodiments, no ex-situ biostimulating nutrient need to be applied to a formation. In particular it is an object of a particularly preferred embodiment of the invention to provide amendments involving manipulation/utilisation of in-situ nutrients preferably such that no, or at least reduced levels of biostimulating nutrients are required. Furthermore, more efficient use of biostimulating nutrients can be measured, for example, in terms of an increase in methane generated per unit of biostimulating nutrient present.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.